The present invention generally relates to radio communications systems and more particularly, to minimizing the effects of multiple access interference in a Direct-Sequence Code Division Multiple Access (DS-CDMA) communications system.
Direct-sequence Code Division Multiple Access (DS-CDMA) techniques are being applied to cellular and personal radio communication systems. With such an approach, all signals share the same frequency spectrum at the same time. For example, suppose that user i conveys an information symbol b.sub.i by transmitting b.sub.i s.sub.i (k), the product of the information symbol b.sub.i and the code or signature sequence s.sub.i (k). By using different signature sequences for different users, the information symbol of a particular user may be determined by correlating the received signal with the user's known signature sequence. Because these codes are rarely orthogonal, the separation is not perfect, so that signals interfere with one another, thereby degrading performance.
One method for dealing with interference in CDMA systems is to perform some form of subtractive demodulation, where each signal, starting with the strongest, is demodulated then subtracted from the composite received signal. If the correct amplitude and phase of each signal is known, as well as the information symbol value, then subtraction is perfect, and only the particular signal of interest is subtracted. Such an approach is described in A. J. Viterbi, "Very low rate convolutional codes for maximum theoretical performance of spread-spectrum multiple-access channels," IEEE J. Select. Areas Commun., vol. 8, pp. 641-649, May 1990.
In practice, the amplitude, phase and symbol values are either unknown or must be estimated, giving rise to estimation noise which leads to subtraction error. One way of estimating these values is to correlate the received data with the known signature sequence. When subtraction occurs, this approach effectively nulls out all components of the received signal that are aligned with the signal being removed, including a small amount of all the other signals. This form of subtractive demodulation is described in U.S. Pat. No. 5,151,919 to Dent which disclosure is incorporated here by reference. Parallel as well serial subtraction is possible.
Because the amplitude, phase, and symbol estimation are noisy, subtraction error occurs, which can accumulate to the detriment of performance. In particular, it has been observed that when the second signal is removed, noise appears in the resultant signal that is aligned with the first signal that was removed. Similarly, when the third signal is removed, noise appears that is aligned with the first two signals. Thus, there is a need for a way to eliminate or minimize the noise introduced by the subtraction process.